1. Field of the Invention
The invention relates generally to the field of marine seismic data acquisition equipment. More specifically, the invention relates to structures for a marine seismic streamer and structures for mounting sensors therein.
2. Background Art
Marine seismic surveying is typically performed using “streamers” towed near the surface of a body of water. A streamer is in the most general sense a cable towed by a seismic vessel. The cable has a plurality of seismic sensors disposed thereon at spaced apart locations along the length of the cable. The seismic sensors are typically hydrophones, but can also be any type of sensor that is responsive to the pressure in the water, or to changes therein with respect to time. The seismic sensors may also be any type of particle motion sensor or acceleration sensor known in the art. Irrespective of the type of such seismic sensors, the sensors generate an electrical or optical signal that is related to the pressure related or motion related parameter being measured by the sensors. The electrical or optical signals are conducted along electrical conductors or optical fibers carried by the streamer to a recording system. The recording system is typically disposed on the seismic vessel, but may be disposed elsewhere.
In a typical marine seismic survey, a seismic energy source is actuated at selected times, and a record, with respect to time, of the signals detected by the one or more sensors is made in the recording system. The recorded signals are later used for interpretation to infer structure of, fluid content of, and composition of rock formations in the Earth's subsurface. Structure, fluid content and mineral composition are typically inferred from characteristics of seismic energy that is reflected from subsurface acoustic impedance boundaries.
In order to improve the quality of seismic data interpretation, one goal of marine seismic streamer design is to reduce the various forms of noise detected by the seismic sensors. A typical marine seismic streamer can be up to several kilometers in length, and can include thousands of individual seismic sensors. Because of the weight of all of the materials used in a typical marine seismic sensor, because of the friction (drag) caused by the streamer as it is moved through the water, and because of the need to protect the seismic sensors, electrical and/or optical conductors and associated equipment from water intrusion, a typical seismic streamer includes certain features. First, the streamer includes one or more strength members to transmit axial force along the length of the streamer. The strength member is operatively coupled to the seismic vessel and thus bears all the axial loading caused by drag (friction) of the streamer in the water. The streamer also includes, as previously explained, electrical and/or optical conductors to carry electrical power and/or signals to the various sensors and (in certain streamers) signal conditioning equipment disposed in the streamer, and to carry signals from the various sensors to a recording station. The streamer also typically includes an exterior jacket that surrounds the other components in the streamer. The jacket is typically made from a strong, flexible plastic such as polyurethane, such that water is excluded from the interior of the jacket, and seismic energy can pass essentially unimpeded through the jacket to the sensors. A typical streamer also includes buoyancy devices at spaced apart locations along its length, so that the streamer is substantially neutrally buoyant in the water. The interior of the jacket is typically filled with oil or similar electrically insulating fluid that is substantially transparent to seismic energy.
The typical fluid-filled streamer structured as described above is well proven and has been used in the seismic surveying industry for a considerable time. However, there are some drawbacks to the fluid-filled streamer structure described above. One such drawback is leakage of the fluid into the surrounding water when a streamer section is damaged or the outer jacket is cut. This allows water to enter interstices of a streamer cable and could cause electrical failure of components in the streamer. At the same time, the streamer buoyancy is compromised. Because the fluid in the streamer is typically hydrocarbon-based, such as kerosene or light oil, such leakage can cause environmental damage. Damage to the streamer can occur while the streamer is being towed through the water or it can occur while the streamer is being deployed from or retrieved onto a winch on which streamers are typically stored on the seismic tow vessel.
Another drawback to using fluid-filled streamers is that detectable noise can be generated by vibrations resulting from the streamer being towed through the water. Such vibrations can cause internal pressure waves that travel through the fluid inside the streamer, such waves often being referred to as “bulge waves” or “breathing waves.” Such noise is described, for example, in S. P. Beerens et al., Flow Noise Analysis of Towed Sonar Arrays, UDT 99—Conference Proceedings Undersea Defense Technology, June 29-Jul. 1, 1999, Nice, France, Nexus Media Limited, Swanley, Kent. Noise in the form of pressure waves can be detected by the seismic sensors, making identification of reflected seismic energy in the recorded signals more difficult.
Still another drawback to fluid-filled seismic streamers known in the art is transient motion of the various components of the streamer. Transient motion can induce detectable noise in the streamer. Ideally, during a seismic survey the entire streamer would move through the water at substantially constant velocity, and all the streamer components (i.e., the outer jacket, connectors, spacers, strength members, and filling fluid) would also move at the same constant velocity and thus not move with respect to each other. Under actual seismic survey conditions, however, motion of the seismic streamer is not uniform throughout and thus can lead to transient motion of various components, most notably the strength members. Transient motion can be caused by events such as pitching and heaving of the streamers, strumming of towing cables attached to the streamers (the strumming caused by vortex shedding on the cables), and operation of depth-control devices located on the streamers.
Transient motion of the strength members can cause transient longitudinal displacement of the spacers or connectors, causing pressure fluctuations in the fluid that are detected by the seismic sensors. Pressure fluctuations in the fluid that radiate away from the spacers or connectors can also cause the flexible outer jacket to bulge in and out as a traveling wave, giving this phenomenon its name. So called “bulge waves” can be detected by the seismic sensors. Another type of noise that can be caused by transient motion of the strength members will be further discussed below.
Other types of noise, generally called “flow noise”, can also affect the signals detected by the seismic sensors. For example, vibrations in and along the seismic streamer can cause extensional waves in the outer jacket and can cause resonance transients to travel along the strength members. A turbulent boundary layer created around the outer jacket of the streamer by the act of towing the streamer in the water can also cause pressure fluctuations in the fluid filling the streamer.
In fluid-filled streamers, extensional waves in the jacket, resonance transients, and turbulence-induced noise are typically smaller in amplitude than bulge waves. Bulge waves are usually the largest source of vibration noise because these waves travel within the fluid core material filling the streamer and thus act directly on the seismic sensors. Nonetheless, all of these noise sources cumulatively can affect the detection of reflected seismic energy from the Earth formations below the water bottom, and thus affect the quality of seismic surveys.
Several methods and structures for streamers have been devised to reduce the foregoing types of noise. One such structure includes compartment isolation blocks within a fluid-filled streamer section to stop the vibration-caused bulge waves from traveling continuously along the entire length of the streamer. Another such noise reducing structure includes open-cell foam disposed in the interior of the streamer. The open-cell foam restricts the movement of the fluid in response to transient pressure changes and causes transient pressure energy to be dissipated into the outer jacket and the foam over a shorter longitudinal distance. Another structure used to reduce noise includes combining (summing) the signals from several longitudinally spaced apart seismic sensors to attenuate effects of relatively slow-moving bulge waves or similar noise. In such structures, an equal number of seismic sensors are positioned between or on both sides of each of the spacers in a streamer segment so that longitudinally equally spaced apart (from the spacer) pairs of seismic sensors detected equal yet opposite polarity pressure changes. Summing the signals from all the sensors in such a group can thus effectively cancel some of the noise.
Another approach to reducing the effects of bulge waves is to eliminate the fluid from the streamer sections entirely, so that no medium exists in which bulge waves can propagate. This approach is exemplified by so-called “solid” streamers, in which each streamer section is filled with a solid core material instead of a fluid. However, in any solid material, some shear waves will develop, which can increase some types of noise detected by the seismic sensors. Shear waves, of course, for the most part cannot propagate in a fluid filled streamer because fluids have substantially zero shear modulus (at least as compared with typical solid materials). Additionally, many conventional solid core materials are not substantially acoustically transparent to pressure waves, thus reducing the sensitivity of such streamers to reflected seismic energy. To deal with the foregoing limitations of using solid fill material in a streamer, another approach to reducing noise in streamers has been developed, which is to replace the fluid with a semi-solid or gelatin-like filler material. Such semi-solid filler material is flexible and acoustically transparent to seismic energy.
It is desirable to have a seismic streamer that has reduced amplitude of compressional waves resulting from tension transients, and that has reduced sensitivity to longitudinally traveling pressure waves in the void filler in the streamer and other types of noise passing along the streamer.